1. Field of the Invention
The present invention relates to a method of bonding a conductive interconnection layer on an electrode of a solid oxide electrolyte, electrochemical cell.
High temperature electrochemical cells are taught by Isenberg, in U.S. Pat. No. 4,490,444. In these type of cells, typified by fuel cells, a porous support tube of calcia stabilized zirconia, has an air electrode cathode deposited on it. The air electrode may be made of, for example, doped oxides of the perovskite family, such as LaMnO.sub.3. Surrounding the major portion of the outer periphery of the air electrode is a layer of gas-tight solid electrolyte, usually yttria stabilized zirconia. A selected radial segment of the air electrode is covered by an interconnection material. The interconnection material may be made of a doped lanthanum chromite film. Suggested dopants are Mg, Ca, and Sr.
Both the electrolyte and interconnection material are applied on top of the air electrode by a modified chemical vapor deposition process, utilizing temperatures of from 1,200.degree. C. to 1,400.degree. C. in a reducing atmosphere, with the suggested use of vaporized halides of zirconium and yttrium for the electrolyte, and vaporized halides of lanthanum, chromium, magnesium, calcium or strontium for the interconnection material, as taught by Isenberg, in U.S. Pat. Nos. 4,597,170, and Isenberg et al., in 4,609,562.
It has been found, however, that there are certain thermodynamic and kinetic limitations in doping the interconnection from a vapor phase by a chemical vapor deposition process at 1,300.degree. C. to 1,400.degree. C. The vapor pressures of calcium chloride, and strontium chloride, are low at vapor deposition temperatures, and so, are not easily transported to the reaction zone at the surface of the air electrode. Thus, magnesium is the primary dopant used for the interconnection material. However, a magnesium doped lanthanum chromite interconnection, for example La.sub.0.97 Mg.sub.0.03 CrO.sub.3, has a 12% to 14% thermal expansion mismatch with the air electrode and electrolyte materials. Additionally, use of halide vapors at 1,300.degree. C. to 1,400.degree. C., in a reducing atmosphere, at partial pressures of oxygen less than 10.sup.-4 atm., can interact with the air electrode material during the initial period of interconnection application. This causes, in some instances, air electrode leaching of main constituents, such as manganese, into the interconnection material, which can cause possible destabilization effects.
In an attempt to solve some of these problems, Isenberg et al., in U.S. Pat. No. 4,598,467, suggested applying a separate, vapor deposited interlayer of, for example, calcium and cobalt doped yttrium chromite, about 1 micron thick (0.001 millimeter), between the air electrode, and the interconnection material and electrolyte. Ruka, in U.S. Pat. No. 4,631,238, in an attempt to solve interconnection thermal expansion mismatch problems, taught cobalt doped lanthanum chromite, preferably also doped with magnesium, for example LaCr.sub.0.93 Mg.sub.0.03 Co.sub.0.04 O.sub.3, as a vapor deposited interconnection material, using chloride vapors of lanthanum, chromium, magnesium, and cobalt. Component oxides, and other chemical forms which decompose to oxides upon heating, such as carbonates, oxalates, formates, and hydroxides, can also be mixed, pressed at approximately 352.5 kg./cm..sup.2 (34.475 MPa-Mega Pascals) and then sintered in an oven at approximately 1,450.degree. C. to form bars of the material.
None of these solutions, however, solve all the potential problems of thermal expansion mismatch, Mn leaching from the air electrode, and the limitations of the incorporation of dopants such as calcium, strontium, and other materials such as cobalt and barium by vapor deposition, in a simple and economical fashion. Many of these problems appear to be associated with the chemical vapor deposition process itself.
Attempts to densify La.sub.1-x Sr.sub.x CrO.sub.3, using solid state sintering, to form an electrode structure, are discussed by Groupp et al., J. Amer. Ceram. Soc., Vol. 59, No. 9-10, pp. 449-450 (1976). They noted that the material was difficult to fabricate by normal sintering techniques, primarily due to volatilization of Cr oxide compounds in oxidizing atmospheres. They prepared compositions containing up to 20 mole % Sr by dissolving nitrates of the constituent La, Sr, and Cr cations in a solution of citric acid and ethylene glycol, followed by evaporation at 135.degree. C., to provide a glasslike resin, which was then calcined at 800.degree. C., to provide a La.sub.1-x Sr.sub.x CrO.sub.3 material. Powder samples of this material, with distilled water as binder, were uniaxially pressed, at 2,115 kg./cm..sup.2 (20.685 MPa), to provide discs of 55% to 60% theoretical density, which were then sintered in the temperature range of from 1,600.degree. C. to 1,700.degree. C. for 1 hour, at oxygen activities of from 10.sup.-12 to 10.sup.-11 atm., to provide compacts having maximum densities of 95%+.
Meadowcroft et al., Ceram. Bull., Vol. 58, No. 6, pp. 610-612, 615 (1979), also recognized oxidation and vaporization problems with Sr or Ca doped LaCrO.sub.3 in air at over 1,600.degree. C. They mixed La.sub.2 O.sub.3 and Cr.sub.2 O.sub.3 with SrCO.sub.3, in appropriate amounts, and prefired the mixture in air at 1,400.degree. C. The reacted powder was first uniaxially, and first isostatically pressed and fired at 1,500.degree. C. in air. The influence of substitutions on vaporization rate was studied for: La.sub.1-x Sr.sub.x CrO.sub.3 (0&lt;x&lt;0.2); La.sub.0.8 Sr.sub.0.2 Al.sub.0.5 Cr.sub.0.5 O.sub.3 ; La.sub.0.8 Sr.sub.0.2 Al.sub.0.25 Cr.sub.0.75 O.sub.3 ; La.sub.0.8 Mg.sub.0.2 CrO.sub.3 and La.sub.0.8 Ca.sub.0.2 Al.sub.0.25 Cr.sub.0.75 O.sub.3. The lowest vaporization rate was achieved for the calcium aluminum containing material.
Ruka, in U.S. Pat. No. 4,562,124, teaches a perovskite-like air electrode material which closely matches the thermal expansion characteristics of support tubes and solid oxide electrolytes in fuel cells. These materials are said to be single phase solid solutions. They are made by mixing the component powders, pressing over 70.5 kg./cm..sup.2 (68.95 MPa), and sintering at from 1,400.degree. C. to 1,800.degree. C. for 1 to 4 hours. Materials made include La.sub.0.3 Ca.sub.0.5 Ce.sub.0.2 MnO.sub.3 ; La.sub.0.7 Sr.sub.0.3 MnO.sub.3 ; La.sub.0.7 Sr.sub.0.2 Ca.sub.0.1 MnO.sub.3 ; La.sub.0.35 Ca.sub.0.65 MnO.sub.3 ; La.sub.0.5 Ca.sub.0.5 CrO.sub.3 and La.sub.0.3 Ca.sub.0.5 Ce.sub.0.2 CrO.sub.3. Air electrode application means are described as plasma spraying, and slurry dipping followed by sintering.
Other methods of making lanthanum and calcium chromium oxides have been tried. Alexandov et al., in U.S. Pat. No. 4,035,266 teach melt production of LaCrO.sub.3 ; CaCr.sub.2 O.sub.4 ; and Ca.sub.0.5 Sr.sub.0.5 Cr.sub.2 O.sub.4, under the action of a high-frequency generator, with a working output of 60 kW at 300 kHz. The melt is then cooled, to provide an ingot of the refractory reaction mixture useful for fuel cell cathodes. None of these teachings provide low temperature formation of a lanthanum chromite structural element in an oxygen atmosphere, without pressure application, on high temperature-reduction degradable, fragile, lanthanum manganite air electrode material, in an electrochemical cell. It is an object of this invention to provide such a process.